The invention relates to rotary drill bits for use in drilling or coring holes in subsurface formations, and particularly to polycrystalline diamond compact (PDC) drag bits.
A rotary drill bit of the kind to which the present invention relates comprises a bit body having a shank for connection to a drill string and a passage for supplying drilling fluid to the face of the bit, which carries a plurality of preform cutting elements each formed, at least in part, from polycrystalline diamond. One common form of cutting element comprises a tablet, usually circular or part-circular, made up of a superhard table of polycrystalline diamond, providing the front Cutting face of the element, bonded to a substrate which is usually of cemented tungsten carbide.
The bit body may be machined from solid metal, usually steel, or may be moulded using a powder metallurgy process in which tungsten carbide powder is infiltrated with metal alloy binder in a furnace so as to form a hard matrix.
While such PDC bits have been very successful in drilling relatively soft formations, they have been less successful in drilling harder formations and soft formations which include harder occlusions or stringers. Although good rates of penetration are possible in harder formations, the PDC cutters suffer accelerated wear and bit life can be too short to be commercially acceptable.
Recent studies have suggested that the rapid wear of PDC bits in harder formations is due to chipping of the cutters as a result of impact loads caused by vibration, and that the most harmful vibrations can be attributed to a phenomenon called "bit whirl". ("Bit Whirl--A New Theory of PDC Bit Failure"--paper No. SPE 19571 by J. F. Brett, T. M. Warren and S. M. Behr, Society of Petroleum Engineers, 64th Annual Technical Conference, San Antonio, Oct. 8-11, 1989). Bit whirl arises when the instantaneous axis of rotation of the bit precesses around the central axis of the hole when the diameter of the hole becomes slightly larger than the diameter of the bit. When a bit begins to whirl some cutters can be moving sideways or backwards relatively to the formation and may be moving at much greater velocity than if the bit were rotating truly. Once bit whirl has been initiated, it is difficult to stop since the forces resulting from the bit whirl, such as centrifugal forces, tend to reinforce the effect.
Attempts to inhibit the initiation of bit whirl by constraining the bit to rotate truly, i.e. with the axis of rotation of the bit coincident with the central axis of the hole, have not been particularly successful.
Although it is normally considered desirable for PDC drill bits to be rotationally balanced, in practice some imbalance is tolerated. Accordingly it is fairly common for PDC drill bits to be inherently imbalanced, i.e. when the bit is being run there is, due to the cutting, hydraulic and centrifugal forces acting on the bit, a resultant force acting on the bit, the lateral component of which force, during drilling, is balanced by an equal and opposite reaction from the sides of the borehole.
This resultant lateral force is commonly referred to as the bit imbalance force and is usually represented as a percentage of the weight-on-bit since it is almost directly proportional to weight-on-bit. It has been found that certain imbalanced bits are less susceptible to bit whirl than other, more balanced bits. ("Development of a Whirl Resistant Bit"--paper No. SPE 19572 by T. M. Warren, Society of Petroleum Engineers, 64th Annual Technical Conference, San Antonio, Oct. 8--11, 1989). Investigation of this phenomenon has suggested that in such less susceptible bits the resultant lateral imbalance force is directed towards a portion of the bit gauge which happens to be free of cutters and which is therefore making lower "frictional" contact with the formation than other parts of the gauge of the bit on which face gauge cutters are mounted. It is believed that, since a comparatively low friction part of the bit is being urged against the formation by the imbalance force, slipping occurs between this part of the bit and the formation and the rotating bit therefore has less tendency to process, or "walk", around the hole, thus initiating bit whirl.
(Although, for convenience, reference is made herein to "frictional" contact between the bit gauge and formation, this expression is not intended to be limited only to rubbing contact, but should be understood to include any form of engagement between the bit gauge and formation which applies a restraining force to rotation of the bit. Thus, it is intended to ,include, for example, engagement of the formation by any cutters or abrasion elements which may be mounted on the part of the gauge being referred to.)
This has led to the suggestion that bit whirl might be reduced by deliberately designing the bit so that it is imbalanced, and providing a low friction pad on the bit body for engaging the surface of the formation in the region towards which the resultant lateral force due to the imbalance is directed. Anti-whirl bits of this type are described, for example, in U.S. Pat. No. 4,982,802.
However, there may be circumstances during operation of such a drill bit when the lateral imbalance force is, temporarily, not directed towards the low friction pad. In the case where the lateral force is generated by the engagement between the cutting elements and the formation, for example, the direction of the force may change when the weight-on-bit is reduced or when the bit is lifted off the bottom of the hole while still rotating. In such circumstances the resultant lateral imbalance force, although reduced in magnitude, may be directed towards a region of the gauge of the bit away from the low friction pad or pads, and where cutters are mounted. The engagement of such cutters with the formation may then be sufficient to initiate bit whirl, which may persist after the bit re-engages the bottom of the hole.
Such temporary re-direction of the imbalance force may also occur as a result of vibration while drilling, or as a result of the bit, or some of the cutters, striking harder occlusions in the formation, or as a result of temporary variation in the hydraulic forces acting on the bit. In each case there is a risk of bit whirl being initiated.
It is an object of the invention to inhibit the initiation of bit whirl, in such circumstances, by using secondary elements to limit the extent to which certain cutters on the bit body may cut into the surrounding formation, thereby limiting the "frictional" engagement of those cutters with the formation.
It is also known, in drill bits of the kind first referred to, to provide on the rearward side of at least certain of the preform cutting elements, which may be regarded as primary cutting elements, secondary abrasion elements which are set slightly below (or inwardly of) the primary cutting profile defined by the primary cutting elements. Such an arrangement is described, for example, in U.S. patent specification No. 4718505.
(In this specification, the "primary cutting profile" is defined to mean a generally smooth notional surface which is swept out by the cutting edges of the primary cutting elements as the bit rotates without axial movement. The secondary profile is similarly defined as the notional surface swept out by the secondary elements.)
With such an arrangement, during normal operation of the drill bit the major portion of the cutting or abrading action of the bit is performed by the preform primary cutting elements in the normal manner. However, should a primary cutting element wear rapidly or fracture, so as to be rendered ineffective, for example by striking a harder formation, the associated secondary abrasion element takes over the abrading action of the cutting element, thus permitting continued use of the drill bit. Provided the primary cutting element has not fractured or failed completely, it may resume some cutting or abrading action when the drill bit passes once more into softer formation.
The present invention is based on the realization that the provision of secondary cutting elements may also help to reduce or eliminate bit whirl, provided that the secondary cutting elements are located and arranged in an appropriate manner. The invention therefore relates to the appropriate disposition of secondary cutting elements on a drill bit, not only on an anti-whirl bit of the kind referred to above, thereby to improve its inherent anti-whirl characteristics, but also to reduce or eliminate the tendency to whirl of other drill bits which are otherwise of more conventional design.